Cast iron pipe



Aug. 2s, 1934.

N. F. s. RUSSELL ET A1. 1,971,385

CAST IRON PIPE Filed May 4, 1953 mw, 11p ma www... It WLM gv w ATfOK/VEY Patented Aug. 28, 1934 UNITED STATES lPATENT OFFICE' CAST IRONPIPE Application May 4, `1933, serial No. 669,356

2 Claims.

C Si S Mn P 05. 15 20-2. 00 balance substantially iron.

As examples of iron compounds which we have successfully employed in themanufacture of our new pipes, we will give the following:

c sis Mn P Example l 3.70 2.02 .065 .56 .49 balance substantially iron.Example 2 3.64 1.72 .074 .43 .81 balance substantially iron. Example3.47 2.20 .147 .42 .77 balance substanu tially iron.

The object of our invention is toprovide a centrifugally cast pipe of aniron composition coming within the above specification having a .markedsuperiority over centrifugally cast pipes as heretofore manufacturedfrom such iron compounds,

particularly in the quality of ductility and consequent capacity tosuccessfully resist impact shocks and characterized in that our pipeshave a distinctive novel structure. i

In our co-pending application for Letters Patent, filed January 17,1933, Serial Number 652,160, We have described and claimed as a newarticle of manufacture a centrifugally cast cast iron pipe made fromirons coming within the above specilication and characterizedin that thecylindrical portion of the pipe is essentially made up of an outerannular zone of not less than one-fourth the thickness of the wall ofthe pipe and which zone is preponderantly made up of compacted,interlaced 5o and substantially nondirectional dendritic crystals offerrite and/or pearlite without intervening areas of that columnarstructure normal to the outer surface of the pipe which ischaracteristic of the formation of what is known as a chill, saidinterlacing dendrites being symmetrically disposed b oth longitudinallyand radially in the zone though radially the dendrites have a tendencyto become somewhat coarser and less numerous toward the inner limit ofthe dendritic zone. Said dendritic zone is further characterized in thatit embodies no areas made up of those refractory forms of combinedcarbon constituting what is commonly known as a chill. A furtherdistinctive feature of the outer zone is that the graphitic carbontherein is dispersed among the dendrites in the form of dots` andpatches of graphite as distinguished from plates or flakes such as occurin the inner zone of the pipe. In the said pipes of .pour formerapplication, the inner zone is preponderantly made up of grains offerrite and/or pearlite substantially without the occurrence ofdendritic crystals and having distributed throughout the matrix offerrite and/or pearlite akes or plates of graphite such as arecharacteristic of ordinary gray iron castings as well as what we maycall unformed dots and patches of graphitic carbon such as characterizethe outer dendritic zone of the pipe. It will be understood that areasof iron phosphide and/.or iron phosphide eutectic will occur in bothzones of the pipe, such areas being somewhat more extensive in the innerthan inthe outer zone. It is a further characteristic of the pipesforming the subject matter of our said former application that, as cast,the peri centage of combined carbon inthe outer dendritic 35 zone isnotably less than that in the inner or graphitic zone. y

The pipes described and claimed in our said former application are, asthey come from the` mold freely machinable and highly resistant toimpactshocks and have in other respects distinct advantages over centrifugallycast cast iron pipe as previously manufactured.

The improved cast iron pipe which forms the subject matter of ourpresent application retains and embodies the distinctive zonal structureof the pipes described in our former application `in that our new pipehas an outer dendritic zone free from areas of chill and preponderantlymade up of compacted interlacing and substantially nondirectionaldendrites without intervening areas of that columnar structure normal tothe 4outer Surface of the pipe which is characteristic of the formationof a chill and the inner zone is preponderantly made up of a matrix ofiron grains through which is distributed formed plates or flakes ofgraphite and in that the outer dendritic zone is free from formed.graphite plates or akes and the inner zone substantially free from vdendritic crystals. The essential differences between the old andnewstructures are, that, whereas in the pipes of our former applicationboth the dendrites of the outer zone and the iron grains of the innerzone were, to a considerable extent, pearlitic in character; in our newpipe substantially all of the dendrites and grains in both zones aredistinctly ferritic in character with only occasional occurrence ofminute specks of pearlite. Another, and we believe even moredistinctive, feature of our new pipe lies in the fact that whereas inthe pipe of our former application the grain structure of the iron, bothin the inner and outer zones, is markedly irregular in form and illdened in outline, whereas in our new pipe the grain structure is made upof polyhedral crystals of quite regular form and size and well definedloutline. A further and notable distinction between the pipe of our olderapplication and that which forms the subject matter of this applicationis that while retaining the good qualities of our earlier pipe, our newpipe is markedly superior in ductility and capacity for resistingshocks.

A further distinctive feature of our new pipe as compared with the pipeof our former application, lies in the fact that whereas, as we havestated, the pipe of our former application, as cast, had a distinctlysmaller percentage of combined carbon in its outer dendritic zone thanin its inner graphitic zone; in our new pipe the combined carbon,materially less in the casting as a whole, is practically in the sameproportion in both inner and outer Zones and' constitutes not more than.15% of the mass of the casting.

In-manufacturing our new pipe we preferably cast the viron into the formof a pipe in an externally cooled centrifugal metal mold by the processforming in part "the subject matter of our copending application, filedOctober 19, 1932, Serial Number 638,480. By this process a coating offinely divided dry coating material is, preparatory to the pouring of,the metal into the mold, deposited upon the inner cylindrical face ofthe rotating mold by means of a jet of carrier gas charged withparticles of finely divided dry coating material directed against theinner face of the rotating mold and the molten metal is then poured intothe rotating mold so as to contact with the so coated cylindricalportion of the mold in the formation of the cylindrical portion of thepipe. As pointed out in our said method application, it is, for the bestresults with regardv to the structure of the casting produced, importantthat the molten metal should be poured upon the coated surface of themold very promptly after the application of the coating for the reason,we believe, that the coating as applied is made up not only of theparticles of nely divided dry coating material but also in part ofadsorbed films of the carrier gas, the presence of which, as a componentpart of the coating is of importance as influencing the character of thecasting and which films of gas have a tendency to escape if the coatingis not promptly covered by the molten metal. We have also in ourmethodapplication pointed out that the efliciency of the coating appliedas described is materially affected by the thickness of the coating andthat we have found it undesirable in the application of the coatingmaterial to the -mold ,to use a quantity of the dry finely dividedcoating material in excess of that which, evenly distributed over thecylindrical portion of the mold and compacted thereon, would form acoating of more than .001 in thickness for the reason that coatings ofgreater thickness are more liable to scale off and also because they aremore liable to be broken and displaced by the impact of the molten metalupon the coating, thereby leaving portions of the metal mold unprotectedand bringing about areas of chill in the casting. We have also pointedout that it is desirable in all cases to use as little of the coatingmaterial in forming a coating for the cylindrical portion of the mold asis consistent with the formation of a coating which will eifectuallyprevent the occurrence of chill in the casting and that, with nelydivided ferro-silicon as a coating material, and this is among the bestof the coating materials we have employed to make an efficient coating,is one made up of finely distributed 'ferro-silicon applied in quantitywhich, if evenly and compactlydistributed over the cylindrical surfaceof the mold, would form a coating of approximately .0003 in thickness.

Cast under the conditions described in our said method application andbriefly summarized above, an iron of the composition which we have abovespecified will produce a pipe having the characteristic features of thatdescribed in our application, Serial. Number 652,160, but for theproduction of our new pipe it is necessary that the casting so producedshould be subjected to a heat treatment somewhat above the criticalpoint at which the crystalline structure of the iron employed willchange from the alpha to the gamma structure and then, to avoid thedevelopment of injurious strains in the casting, gradually cooled to atemperature of 1200" F. after which cooling of the casting may becarried on at any convenient speed. The effect of this heat treatmentwhich involves, rst, the change of the alpha structure to that of thegamma structure and then ,the reformation of the alpha structure oncooling below the critical point, is to effect a very marked changeinthe grain structure of the casting in that'the grains which in thecasting as it comes from the mold, are ill dened and of markedlydistorted and irregular form are, after the heat treatment, found to bein the form of well defined polyhedral crystals of marked regularity insize and shape. The heat treatment of the casting necessarily and nodoubt advantageously reduces the combined carbon in the casting to apoint not in excess of .15%v and in practice considerably lower. Thegraphitic carbon thus thrown out of combination is found in the treatedcasting in the form of dots and patches as distinguished from formedplates of lgraphitic carbon and the low percentage of combined carbonexisting in the dendritic zone naturally results in the graphitic carbonso formed being notably smaller in quantity in the outer zone than inthe inner zone, a fact which we believe is highly advantageous inpreserving the increased ductility of the dendritic zone, due to theheattreatment and the reformation of the grains.

The notable effect of the heat treatment upon the casting is not only tobring about the structural changes which we have above noted but alsoto'impart to the cast pipe a very marked'increase in shock resistingcapacity as compared with the improved pipe forming the subject matterof our former application and this marked increase in ductility andshock resisting capacity is secured without impairment of any of thegood qualities of the pipe as cast: for example, an approved method fortesting the shock resist- 5 feet, to drop a 50 pound weight` upon the sosupported portion of the pipe. Noting' thefirst occurrence of a crack inthe pipe, then the first occurrence of a perceptible leak and finallythe occurrence through the crack of the escape of` ysuch a sheet ofwater as would amount to a total failure. Tested in this way, theunannealed pipe generally failed before the delivery of a blow from the5 feet elevation while the annealed pipe very generally showed nofailure even after the delivery of fine blows from a` height of 5 feetand, computing the energy in foot pounds of the blows delivered toproduce failure or, in the case of the annealed pipe, to produce eitherfailure or on the assumption that failure had occurred after thedelivery of ve blows from the upper height of 5 feet, the test showedthat the changes effected in the unannealed casting by the heattreatment, which changes constitute the novel features of our new pipe,had increased the impact resisting quality of the pipe, over that of thepipe as cast, by more than 300%. To obtain a pipe having thedistinguishing structural features characteristic of our new pipe andits notably increased capacity to resist shock, it is necessary that thepipe should be heated to temperatures above the critical point at whichthe change from the alpha to the gamma structure occurs and thiscritical point will varyk somewhat with the composition of the iron but,

periments have led us to the conclusion that even better pipes will beproduced if the maximum temperature to which the pipes are raised islessened so long as care be taken that the casting shall be heated abovethe critical point.

In view of the fact that the cast pipe as it comes from the moldembodies in its cylindrical portion no areas of that refractory form ofcombined carbon which constitutes a chill. It will, of course, beobvious to those skilled in the art that it is not necessary, in orderto reduce the percentage of combined carbon to .15% or less, to heat thecast pipe to as high a temperature as that of the critical point but toobtain the distinctive structure and distinctive qualities of our newpipe it is absolutely necessary that the cast pipe should be heated totemperatures in excess of the critical point.

While we believe that the structure and composition of our new pipe willbe entirely clear to those skilled in the art, from the foregoingdescription, a clearer understanding of the structural peculiarities ofour pipe may be aided by reference to the drawing ,in which Figure 1 isan elevation of our pipe shown, so far as the cylindrical portionthereof is concerned,

in central section and partly broken away in the middle.

Figure 2 is a cross-sectional view taken, for example, on the line 2-2ofFig. 1. y

Figure 3 is a somewhat diagrammatic microphotographic view of a typicalarea of the dendritic zone of the pipe taken on a scale of enlargementvwhich will best display the dendritic structure of this portion of thepipe. A scale of enlargement of 100 diameters we have found to beeffective for .this purpose but it will be understood that this scaleisnot applicable to the drawing as it will appear on the reduced scale ofthe printed drawing forming a part of the patent specification.

Figure 4 is a mcrophotographic view of a typical area of thegraphiticzone of our new pipe taken on a similar scale of enlargement to that ofFig. 3.

Figure 5 is again a microphotographic View of an area of the dendriticzone on a scale of enlargement which` will better 4exhibit the grain'structure of the ferrite constituent of this zone, the enlargement beingon a ratio of '700 diameters as compared with the 100 diameterenlargement of Fig. 3, and

, Figure 6 is a view of a typical area of the graphitic zone shown on asimilar scale of enlargement to that of Fig. 5, to show the grainstructure of the ferrite occurring in this zone of the pipe.

It will be understood, with reference to Fig. 3, that any surface of thedendritic zone prepared from microscopic examination and photographywill intersect the interlacing dendrites, at every conceivable angle andthis is also true as to the intersection of the plane with the graincrystals which we have attempted to illustrate in Fig. 5. This must beborne -in mind also in considering Fig. 6 and, to a certain extent inconsidering Fig. 4, particularly with regard to the intersection of theplane under observation with the formed plates or flakes of graphite.With these facts in mind, our diagrammatic drawing is, we believe,helpfully illustrative as tothe structures which they are intended tomake clear.

In the drawing,

lio

A indicates the bell end and A1 the cylindrical body of our pipe. Bindicates the`outer dendritic zone of our pipe shown as extending fromthe outer surface for about one-third the thickness of the pipe. Cindicates the inner graphitic zone of the pipe. Inl Fig. 3, D indicatescertain of the dendritic:` .crystals of ferrite which appear in the viewin a manner which makes clear their densection under examination of theunformed dots -and patches of graphitic carbon and at F we indicate theoccurrence in the section depicted 0f areas of phosphide eutectic.Referring to Fig. 4, G indicates thegranular ferrite matrix makingl upthe preponderant mass of the graphitic zone of the pipe. H, H, etc., thedistributed formed plates or flakes of graphite characteristic of thisinner zone of the pipe. E, E, indicates the occurrence in this zone ofunformed dots and patches of graphitic carbon similar, to those whichoccur in the dendritic zone and at F, F, we have indicated theoccurrence of areas of phosphide eutectic occurring in this graphiticzone of the pipe. In Figs. 5 and 6, we have indicated the structure ofthe ferrite grains. At I, I, etc., we have indicated the occasionaloccurrence in these grains of very minute specks of pearlite.

We havealso in the drawing indicated the various structural componentsof the illustrated structure by labels.

To sum up the characteristic features of our new pipe, it has thedensity and oompactness of Aing mass of the outer zone are interlacingand without unified directional tendency and are symmetricallydistributed throughout the structure of the outer zone withoutintervening areas of that columnar crystalline structure normal to theouter surface of the pipe which characterizes what is commonly known asa chill. Its inner zone is characterized in having distributed formedplates or flakes of graphite throughout a matrix of ferrite and by thenonoccurrence lin this zone of dendritic formations. The combined carbonin both zones of the pipe does not exceed .15% and does not materiallydiffer in the inner and outer zones. The dendrites of the outer zone andthe grains of theiron in both zones are distinctly ferritic in characterwith only occasional inclusions of fine specks of pearlite. The grainsof the ferrite both in the outer and inner zones are in the form of welldefined polyhedral crystals of regular form and size and, finally, ournew pipe has a greatly increased capacity to resist impact shocks ascompared with the cast pipe as it comes from the mold or withcentrifugally cast castiron pipe as heretofore produced.

Having now described our invention what we claim as new and desire tosecure by Letters Patent, is: f-

1. An annealed centrifugally cast cast iron pipe, the cylindricalportion of which is freely machinable and highly resistant to impactshocks and has a composition coming within the following specication:

balance substantially iron, said pipe having as cast an outer annularzone of substantial depth preponderantly made up o-f compactedinterlaced dendrites of ferrite and/or pearlite without unifieddirectional tendency and symmetrically distributed throughout the lengthand depth of said zone, said outer dendritic zone being furthercharacterized in that it is free from distributed formed plates ofgraphite and from areas of chill, said cylindrical portion of the pipebeing further made up as cast of an inner annular zone in which formedplates of graphite are distributed throughout a matrix of ferrite and/orpearlite grains substantially free from dendritic formations, said castpipe being heat treated after casting by raising its temperature to apoint above that critical point at which a change from the alpha to thegamma structure occurs in the crystalline grains of the iron componentof the casting and then permitted to cool and characterized after suchheat treatment in that the dendrites of the outer zone and the ironmatrix of the inner zone are made up of grains of ferrite substantiallyfree from pearlite and in that said grains are preponderantly made up ofclearly defined and symmetrical polyhedral crystals and further in thatthe shock resisting capacity of the pipe is materially increased ascompared with the pipe as cast'.

2. An annealed centrifugally cast cast iron pipe having thecharacteristics called for in claim 1, and in which the 'outer dendriticzone comprises not less than onefourth of the thickness of the pipe.

`NORMAN F. S. RUSSELL.

FREDERICK C. LANGENBERG.

